Connection event type checkboxes are horrid and confusing

[Half-Shot]

Currently, we have checkboxes like so in our widget:

The problem with these checkboxes, is that they are confusing and don't conform to what people would expect them to do.

• For instance, checking the top level "issues" checkbox will enable all issue events by adding the "issue" type to your allowed list.
• However, disabling it but leaving all the child items ticked will do the same thing.

I think generally it would be better to do away with the root checkbox items entirely, and instead move to a model where we just explicitly opt into each and every feature, rather than having parent and child event types.

[AndrewFerr]

To tack onto this, trying to un-tick a child item whose parent item is checked has odd results:

https://github.com/matrix-org/matrix-hookshot/assets/3271094/7b3e066d-2fcb-4a21-b9c1-77d4668162bd